// ProblemSet.java
import java.util.ArrayList;
import java.util.List;

public class ProblemSet {
    private List<Problem> problems;

    public ProblemSet() {
        problems = new ArrayList<>();
    }

    public void addProblem(Problem problem) {
        problems.add(problem);
    }

    public void displayProblems() {
        for (Problem problem : problems) {
            problem.display();
        }
    }

    public int size() {
        return problems.size();
    }
}